pleo

Etymology
Exists only as a bound morpheme in prefixed verbs, where it continues, from earlier *plējō, from , a yé-present innovatively formed to the aorist stem of. Compare 🇨🇬, a cognate with the same meaning that likewise does not occur uncompounded.

Conjugation
Only one form, (CIL 2.6278.38), is attested in classical Latin without a prefix outside of grammarians. (Festus p.230M) also cites without attribution.